A common example of a simple SLM is a liquid crystal computer display. In this, a two-dimensional array a of liquid crystal cell is uniformly illuminated and the transmission of each cell is controlled to from a display on a screen. In a black and white display, each cell corresponds to a pixel in the image.
However, when the illumination is not uniform, such as from another image, an SLM can perform other functions. For example, a correlation can be obtained between the illuminating image and a second image. This is accomplished by driving the SLM with control voltages that would produce the second image if the illumination were uniform. The SLM output from each cell is the product of the pixel amplitude in the illuminating image and the second image. By collecting and measuring the light from the entire SLM output, the correlation between the two images can be obtained.
Besides the simple example, SLMs have been proposed for use in a variety of diverse applications such as radar signal processing, oil field exploration, weather prediction, air flow simulations, image storage and processing, holographic video systems, large database storage, and optical buses between cache memories in multiprocessors.
However, so far, only liquid crystal cells have been used. These materials have the disadvantage of being inherently slow, especially at reduced temperatures that might be encountered outside a laboratory environment. Thus, there is a need for materials with a faster response times.
One of the challenges in making SLMs is the need to individually control a large number of cells. In computer displays, the approach is to use orthogonal transparent strip electrodes on either side of the liquid crystal cells so that each cell is at the intersection of an X-address line and a Y-address line. Individual cells are controlled by timing voltages on selected address lines. In this approach, a sheet of glass is coated with X-address lines and another with Y-address lines and the liquid crystal material is sandwiched between. However, this approach is not suitable for the solid state materials that have faster response times.
This invention is in the field of amplitude modulation of light. More particularly, it provides a two-dimensional array of amplitude modulators or a spatial light modulator (SLM) using solid state materials formed using a ceramic-on-silicon fabrication process, for example.
Accordingly, the present invention provides a two dimensional pixellated device that uses a material that has an inherently fast response time as an optical switch. By forming the pixellated array on an integrated circuit, a system is provided for controlling the array that is compatible with existing digital signal processing computers.
A silicon CMOS integrated circuit (IC), having random access memories (RAMs), for example, that has been fabricated in a substrate and interfaced to solid state electro-optic materials positioned thereon illustrate a preferred embodiment of the invention. In a particular embodiment, the electro-optic modulators are controlled by RAM cells to produce a modulation in reflected light incident on the device. SRAMs can be used with a connection to the SRAM cell flip-flop. DRAMs can be used with the modulator replacing the DRAM storage capacitor. The SLM thus formed can be connected to a digital computer and controlled as if were a being written to as a memory, but other IC structures can also be used. In order to enhance the modulation effects, the electro-optic material is used as the spacer for a Fabry-Perot etalon structure that is also deposited on the RAM substrate.
A solid state material such as lead lanthanum zirconate titanate. (PLZT) is a suitable electro-optic material. Proper proportioning of the elements in such a material can be used to avoid thermal mismatch of the material and the substrate. A sequence of layers of the solid state material can be deposited in the liquid phase and heated to provide a sufficiently thick layer without thermal mismatch to the existing substrate. The modulator array is then interconnected to the integrated circuit.